Axial-flow fan units include rotating, airfoil-based rotor blades in which the working fluid (e.g., air or other gas) principally flows parallel to the axis of rotation. Axial-flow fans, and particularly axial-flow ducted fans, are typically used in turbomachinery, such as jet engines, high-speed ship engines, and small-scale power stations. They are also used in industrial applications such as large-volume air separation plants, supplying blast furnace air or fluid catalytic cracking air, and propane dehydrogenation. Additionally, axial-flow fans can be used in cooling fans for homes, automobiles, locomotive vehicles, buses, marine vehicles, and aircrafts such as vertical and short take-off and landing (VSTOL) vehicles and uninhabited aerial vehicles.
In some situations, there can be inlet flow separation at the inlet of the axial-flow fan. For example, inlet flow separation can occur when the inlet flow direction is misaligned with the rotational axis of the axial-flow fan system. Conventional axial-flow fans are often implemented in VSTOL vehicles to generate the lift force required for hover-type flight. For example, an axial-flow fan, such as an axial-flow ducted fan, can generate a downward-facing force required for hover-type flight. As shown in FIGS. 1A-1C, a conventional fan 1000 includes a hub 100 having a front portion 101 and an end portion 115, a rotor 105 having a plurality of blades 107 rotatably coupled to the front portion 101 of the hub 100, and a duct 110 circumscribing the blades 107 of the rotor 105. In the particular embodiment illustrated, the duct 110 is a cylindrical structure that surrounds the blades 107 of the rotor 105. The duct 110 is coupled to the hub 100 by a plurality of outlet vanes 109. In between the front portion 101 of the hub 100 and the duct 110 is the inlet 125. The inlet 125 includes an inlet lip 119 proximate to the leading edge of the duct 110. The illustrated conventional axial-flow fan 1000 has a single duct 110. During horizontal flight, cross-wise air flow (e.g., air flowing perpendicular to the rotational axis of the rotor 105) encounters the outer surface of the duct 110. The cross-wise air flow separates between the top and the bottom of the duct 110. The air flow that travels across the leading edge of the duct 110 and past the inlet lip 119 enters the inlet 125. However, as air flow enters the inlet 125, there can be flow separation at the inlet lip 119 of the duct 110. This distortion will be referred to herein as lip separation. The cross-wise air flow is moved and forced through the inlet 125 by the rotating blades 107 of the rotor 105. The air flow exits the duct 110 at an outlet 126 beneath the outlet vanes 109.